It is known that transesterification of vegetable oils with an alkanol has a decisive influence on the quality of fuels produced from the oils, because the transesterification reaction determines whether the viscosity of the resulting fuel suitable for use in fuel-injection engines.
Transesterification of vegetable oils with an alkanoi proceeds in a reversible, equilibrium reaction according to the following scheme:
in the above formula, R, R′ and R″ stand for the hydrocarbyl moieties of the fatty acid constituents of vegetable oils and Alk is a C1-C4 alkyl group. As shown in the above scheme, glycerol is formed as a by-product in the reaction in addition to the fatty acid methyl esters, which are usable as fuel. The equilibrium can be shifted towards the formation of the required fatty acid alkyl esters by increasing the amount of the alkanol reactant and/or by removing the glycerol by-product. Transesterification is performed generally in the presence of a catalyst. Usually bases are applied as catalysts (most frequently potassium hydroxide); acid catalysts are used less frequently. Methanol is the most frequently used alkanol.
A standard method for transesterifying vegetable oils using methanol and for producing biodiesel fuel are disclosed in U.S. Pat. Nos. 2,360,844, 2,383,632 and 2,383,633. This method has been generally used, with some minor modifications, for the production of biodiesel fuel on industrial scale until now. According to the method disclosed in the cited references, vegetable oil is reacted with methanol in the presence of potassium hydroxide catalyst for at least one hour at a temperature below the boiling point of methanol (usually at 65° C.). As methanol and vegetable oil have only limited miscibility in each other, the biphasic reaction mixture is intensely stirred and/or phase transfer catalysts are used in order to accelerate the reaction. Glycerol, which is liberated as a by-product, accumulates in the polar (methanol) phase, and—in accordance with the equilibrium nature of the reaction—is prone to reconvert the once-produced fatty acid methyl esters into glyceride esters. The optional phase transfer catalyst also contributes to this reconversion process, thus no full conversion of the vegetable oil can be attained. When the reaction mixture is close to equilibrium, which corresponds to a conversion rate of about 80%, the mixture is allowed to settle for about one day. Thereafter, the lower polar phase (which comprises glycerol together with the major portion of methanol used in excess) is removed, and the reaction is repeated with the upper apolar phase with freshly admixed alcohol and catalyst. After this second step, the upper apolar phase (fuel phase) is separated again, subjected to distillation to remove part of the methanol contained therein, washed with aqueous sulphuric acid to remove potassium hydroxide, washed again with water, dried and filtered (this latter series of operations is to refine the biodiesel fuel). If desired, quality improving additives, particularly additives improving cold resistance, are added to the resulting biodiesel fuel.
Major disadvantages of the above method are as follows: due to the biphasic mixture, the reaction requires a lengthy period of time and an energy consuming intense stirring to proceed; separation of the phase which contains the glycerol by-product is difficult and extremely time consuming; methanol is used in a great excess to the stoichiometric amount in order to shift the reaction towards the formation of the required product; the majority of non-reacted methanol appears in the glycerol phase from which it cannot be recovered in an economic way unless acceptable operational capacity is employed. In a paper discussing the improvement potentials of biodiesel transesterification fuel production (D. Darnoko and M Cheryan: JAOCS 77, 1269-1272 (December, 2000)), the authors mentioned further disadvantages in that the method requires large reactor volumes and repeated start up/shut down cycles, resulting in increased in capital and labour investments and decreased in production efficiency, and furthermore, the quality of the product may vary from batch to batch.
According to U.S. Pat. Nos. 5,520,708 and 6,015,444, the time required for transesterification and phase separation is reduced by performing the reaction in an assembly of a static mixer, a heat exchanger, a homogenizer and a settling vessel rather than using a conventional reaction vessel equipped with a stirrer. The static mixer used in this method, like all static mixers, does not contain moving means for mixing the reactants; a turbulent flow created either by flow-breaking means (such as baffles, ribs, coils etc.) mounted inside the mixer or by a packing filled into the mixer serves to mix the reactants intensely. Vegetable oil, alkanol (most frequently methanol) and catalyst are passed through the static mixer, and the resulting dispersed stream is heated to reaction temperature in the heat exchanger. Thereafter the mixture is subjected to a high shear in the homogenizer to form an emulsion, and the emulsion is passed into the settling vessel where no further stirring is applied. The lifespan of the emulsion formed in the homogenizer enables transesterification to proceed to equilibrium conversion before the emulsion segregates in the settler. Although a significant reduction in time requirement can be attained with this method, it has a disadvantage in that a homogenizer with high energy consumption is used. Again, a conversion rate higher than the equilibrium in a single step may not be attained by using this method. Thus, in order to attain 95-98% conversion rate, which is required to obtain a fuel product with appropriate viscosity, the apolar phase needs to be reacted once again after the first step. A great excess of alkanol needs to be used, which is unacceptable for industrial scale production.
According to the method disclosed in DE 42,09,779, 98% conversion of vegetable oil is attained by performing transesterification in a column divided into reaction zones and separation zones, wherein each reaction zone is followed by a separation zone. The vegetable oil, the alkanol and the catalyst are fed into the first reaction zone. After a prescribed period of time, the resulting mixture is fed into the first separation zone and the glycerol is removed by centrifugation. The resulting glycerol-free mixture is then fed into the second reaction zone, and the above reaction/separation steps are repeated in series until the required rate of conversion is attained. An important advantage of this method is that it can also be performed as a continuous operation, because the time consuming step of settling the mixture is replaced by the much faster step of centrifugal separation. This advantage is, however, overcompensated by the extremely high installation costs which render the method too expensive. Therefore, this method is not employed in plants which have a capacity lower than 100,000 tons/year. As a further disadvantage, this method cannot be applied for transesterifying vegetable oils containing more than 2% of free fatty acids.
In order to avoid the disadvantages associated with heterogeneous reactions, particularly in order to decrease reaction time and energy demand of stirring, a method to transesterify vegetable oils with alkanol under homogeneous reaction conditions has been suggested. [www. bioxcorp.com with reference to Production of a cost-competitive biodiesel fuel alternative to petroleum diesel, in Environmental Science & Engineering, May, 2001.]. According to this method, a polar solvent that is highly soluble both in the polar alkanol and in the apolar vegetable oil (such as tetrahydrofuran or N-methyl-2-pyrrolidin-one) is utilized as a reaction medium. However, such a method requires very complicated and energy consuming separation steps for processing the final reaction mixture, which overcompensates the advantages resulting from the use of a homogeneous mixture. Specifically, due to a change in phase conditions after transesterification, the polar solvent is distributed between the biodiesel phase with increased apolarity and the glycerol phase with increased polarity and needs to be removed from both phases. The disadvantages arising from the equilibrium nature of the reaction cannot be avoided with this method, since glycerol is continuously present in a reactive state in the reaction mixture for transesterification. Thus, this solvent assisted method has not been utilized on an industrial scale.
Publications relating to biodiesel fuel production generally state that efficient transesterification, which is imperative for obtaining a product having the required quality, requires the use of refined vegetable oil as a starting substance. This is particularly important when the vegetable oil is a waste (e.g. used frying oil). No specific method for refining vegetable oils has been disclosed, however, in publications relating to biodiesel fuel production. According to the known technologies for producing food grade products, vegetable oils are refined by treating them with water to remove hydratable phospholipids and with a cids, such as phosphoric acid or citric acid, to remove non-hydratable phospholipids [Hoffman, G.: Chemistry and technology of edible oils and fats and their high fat products, Academic Press, London ; Toronto, 1989]. The resulting refined vegetable oils (termed in food technology as “degummed oils”) are suitable to be used as starting substances for biodiesel fuel production. From the aspect of transesterification proceeding in heterogeneous phase, it is an advantage that such refined (degummed) vegetable oils contain a series of minor components having some surfactant properties. In the method of U.S. Pat. Nos. 5,520,708 and 6,015,444, these vegetable oil components are utilized to form an emulsion. However, the difficulties in the separation of the apolar phase comprising transesterified substances from the polar phase comprising glycerol by-product can be attributed to the presence of these components.
It would therefore be desirable to provide improvements in transesterification, which is a key step in biodiesel fuel production. The present invention provides a method which enables one to reduce considerably the time requirements of transesterification and subsequent glycerol removal without requiring expensive equipment or complicated processing steps. The present invention further provides a method which enables one to reduce considerably the alkanol requirement of the transesterification reaction without affecting the conversion rate and to attain the required conversion rate of 95-98% without the time-consuming intermittent glycerol separation steps.